You duel a Freeze - Try and get as far away as possible while shooting at them.

You duel a Thunder- Use the Thunder's splash damage against it to weaken it for your own guns.

You encounter a heavy tank - If you run into a heavy tank, eg Titan (Especially M3) or Mammoth, ram them. They won't tip, but will be stuck at an angle and be unable to hit you- Unless they have Rico due to its angle of fire. I use Mammoth, and found this to be very annoying.

You duel a Railgun or Shaft- Make them miss their shot. Less likely to work against Titan or Mammoth due to weight.

PAIRINGS

Twins and Hunter- Good because they are unlocked very close to each other. Are cheap and both are very easy to upgrade to M3, as Hunter M0-3 costs only 990 crystals.

Twins and Viking- An overused pairing. You can't go a game without seeing one of these. Probably the best paring as the Viking is cheap for M2 at least. Speed counters the Railgun, Thunder and Shaft Players shooting at you from a distance, as you can reach them quickly.

Twins and Hornet- A mixed bag. Does not last long against other Twins. However you can zoom around large tanks eg Mammoth and Titan, and if they have Thunder, Shaft, or Railgun, will have a hard time hitting you. I know because I tried to kill a Hornet doing this with my Thunder and Mammoth and ended up in pieces.